GB2139295A

GB2139295A - Thermal joint e.g. for a turbine

Info

Publication number: GB2139295A
Application number: GB08410380A
Authority: GB
Inventors: Tuomo Kaivola
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-05-05
Filing date: 1984-04-24
Publication date: 1984-11-07
Also published as: GB8410380D0

Abstract

The joint comprises two parts 1, 2 of identical cross sectional shape a1, b1, c1, d1, e1, f1, and a2, b2, c2, d2, e2, f2 respectively, the two parts being engageable with each other to form a structure having a locking cavity c1, d1, d2, c2. The parts 1, 2 have an equal thermal expansion co-efficient, and the locking cavity c1, d1, d2, c2 contains a locking member 3 having a larger or a smaller thermal expansion co-efficient than that of the parts 1, 2. The joint may be used to mount turbine blades to their rotor or in the shroud band surrounding the tips of the blades. <IMAGE>

Description

SPECIFICATION A thermal joint and a self-corrective turbine structure including the thermal joint This invention relates to a thermal joint and a self-corrective turbine structure including the thermal joint. The turbine structure may be a turbine rotor and especially the rotor of a gas turbine. The turbine structure may also be any part of the turbine giving anti-vibrational and/ or damage tolerant structures.

It is known that heat, vibration and blade disengagement are some of the main problems besetting turbines. These problems are particularly acute with gas turbines. Various solutions have been proposed and they have the common feature of treating the rotor assembly as a basically rigid body to which devices such for example as vibration damping devices are attached. In British Patent Specification No. 1503453, it is recognised that at least for anti-vibrational purposes, the most advantageous structure is a continuously discontinuous one that is formed by a continuously repeated joint. In Specification No.

1503453, the joint is incapable of producing a connectable assembly and therefore a connectable assembly is attained through the joining of a blade and a central disc. The joining of the blade and the central disc are regarded in the known solutions to the abovementioned problem as being a rigid assembly.

Furthermore, Specification No. 1 503453 does not relate to gas turbines.

With regard to problems caused by heat, metallurgical solutions have hitherto mainly been contemplated. There has been no attempt to combat the solution by providing a new turbine design, least of all a new turbine design that is applicable to gas turbines. However, it is in gas turbines that the problems caused by heat are most acute because of the great temperature variations in gas turbines.

In Finnish Patent Specification No. 25428, there is shown a self-engaging joint which is for joining beams in timber structures.

The present invention aims to use the joint structure of Finish Patent Specification No.

25428 in a structure which forms a kind of thermal mechanism and to provide a turbine structure which is designed to be capable of a regulated response to vibration and to shredding forces created by thermal expansion and contraction.

Accordingly, this invention provides a thermal joint comprising two parts (1, 2) having identical cross-sectional shapes (a1, b,, c1, d1, e1, f1 and a2, b2, c2, d2, e2, f2), the two parts being engageable with each other to form a structure having a locking cavity (c1, d1, d2, c2), the parts (1, 2) having an equal thermal expansion co-efficient, and the locking cavity (c1, d1, d2, c2) containing a locking member (3) having a larger or smaller thermal expansion co-efficient than the thermal expansion co-efficient of the parts (1, 2).

This invention also provides a turbine structure comprising a thermal joint of the invention.

The turbine structure may be a turbine rotor. The turbine rotor may comprise a central disc and a circumferential set of blades, the blades engaging each other to form a selfclosing circumferential chain and being joined by joints of the invention or the blades being connected to the central disc by joints of the invention.

The turbine rotor may consist of a central disc (6) and a set of blades (5), the central disc (6) having a rim profile with a crosssection (a1, b1, c1, d1, e1, f,) as desired of a member of the thermal joint, the root portions of the blades (5) having complementary shapes, and the blades (5) being positioned on the circumference of the central disc creating by repetition a circumferential continuous thermal joint with the central disc.

The present invention also provides a structural chain forming a continuous rigid body consisting of a chain of components engaging each other by the thermal joint of the invention, the joint being continuously repeated.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a basic joint structure; Figure 2 is a perspective view of parts of the basic joint structure; Figure 3 is a section through a structure; Figure 4 is a section through a structure; Figure 5 is a section through a structure; Figure 6 is a section through a structure; Figure 7 is a section through a structure; Figure 8 is a section through a structure; Figure 9 is a side view of a structure; Figure 10 is a side view of a structure; Figure 11 is a perspective view of a part of the joint; Figure 1 2 is a side view of part of a rotor assembly; Figure 1 3 is a perspective view of part of a rotor assembly;; Figure 1 4 is a perspective view of a structure; Figure 1 5 is a section through a structure; and Figure 1 6 is a section through a structure.

Referring to Fig. 1, there is shown a basic joint structure. Parts 1 and 2 are formed to have a shape (a1, b1, c1, d1, e1, f1 and a2, b2, c2, d2, e2, f2) which enables the parts 1 and 2 to be joined together. When the parts 1 and 2 are so joined together, a locking cavity (c1, d1, d2, c2) is formed within the structure. If a locking member 3 is placed in the locking cavity, the parts 1 and 2 remain locked together. If the member 3 has a higher thermal expansion co-efficient than the thermal expansion co-efficient of the parts 1, 2 then, when the basic joint structure becomes warm, the member 3 expands faster than the locking cavity and there results a pressure in the locking cavity.

The behaviour of the basic joint structure depends on the value of the angle a formed by the lengthwise direction (d, e) of the joint and the locking oblique surfaces (e, f). If the thermal expansion co-efficient of the parts 1, 2 is k1, and the thermal expansion co-efficient of the member 3 is k2, then k may be designated as the difference k2 - k1 of the thermal expansion co-efficients. If now the longitudinal dimension c1, d2 (c2, d1) of the locking cavity is q, then the force F1 acting in the lengthwise direction against each point of the transverse surfaces c1, d, (c2, d2) of the locking cavity is kq. If the transverse dimension c1, d, (c2, d2) of the locking cavity is h then the force F2 acting in the transverse direction against the lengthwise surfaces c1, d2 (c2, d,) at each point is kh.The total pressure in the lengthwise direction of the structure is h . kq. The total pressure in the transverse direction is q . kh. Thus the total pressure forces in the lengthwise and transverse directions are equal irrespective of the proportions of the locking cavity.

If the angle a formed by the lengthwise direction (d, e) and the locking oblique surfaces (e, f) is 45t, then the lengthwise and transverse forces are at equilibrium. Therefore, in order that the structure should have a tendency to tighten rather than to burst, the angle a must be smaller than 45 , providing that the member 3 has an equal thermal expansion co-efficient in all directions, the shape of the locking cavity is rectangular, and the surfaces of the locking cavity are planes.

A self-tightening joint is then produced as the structure gets warm. If the member 3 has a smaller thermal expansion co-efficient than the parts 1, 2 then a self-tightening thermal joint is produced by the structure cooling.

It is to be appreciated that whilst the following description refers to the member 3 having a thermal expansion co-efficient which is greater than that of the parts 1, 2 for the purposes of producing a self-tightening thermal joint obtained as the structure warms up, the reverse situation is equally applicable in which the thermal expansion co-efficient of the member 3 is smaller than the thermal expansion co-efficient of the parts 1, 2 so that a self-tightening thermal joint is produced by the structure cooling down.

The requirements concerning the value of the angle a can be varied as a result of different thermal expansion co-efficients in different directions of the member 3 and can thereby produce greater possible values for the angle a. In order to retain the members 3 in place, the transverse surfaces c,, d, (c2, d2) can have a faceted or curved concave surface (c1, c111, C1/, d1, dl", dl') instead of being plane surfaces, see Fig. 2. If now the angle contained in this surface is Y, then the thermal expansion of the member 3 in a direction s produces a vector component in the longitudinal direction d, e, which component concerning both slopes is (s/2)2 cot (Y/2) cos (Y/2) sin (Y/2).Thus, for the value of a, it follows that (hq + 2(s/2)2 cot (Y/2) cos (Y/2) sin (Y/2)) cos a > hq sin a, where s is the dimension perpendicular to the cross-sectional plane (abcdef), in order for the thermal joint to be formed.

If the longitudinal surfaces d, e have a convex curvature in the direction s, then the greater thermal expansion co-efficient of the member 3 results in a pressure against the longitudinal surfaces d, e decreasing towards zero. The possible values of the angle a in this case follow from the general conditions of stability of the joint structure. The angle a must be smaller than 90 . However, the angle a must also be smaller than the angle which is formed by the longitudinal direction d, e of the joint and that tangent which goes through the point e and is a tangent of that circle whose centre is the point where the other oblique locking surface e, f and the outside surface of the joint body intersect, and which goes through the point e.

If desired, the member 3 may be arranged to have different thermal expansion co-efficients in different directions by forming the part 3 of longitudinal slices 3, and 32, see Fig. 4, with differing thermal expansion co-efficients k21 and k2 2 Then, the thermal expansion coefficient in a longitudinal direction d, e is greater than the composite thermal expansion co-efficient in the transverse direction c1, d, (c2, d2).If then the respective transverse dimensions of the assemblages of such longitudinal slices 3, and 32 are h, and h2, then, for the angle a, it holds that if k21 > k22, and that if k2, rel = k21 - k1 and k22,e, = k22 - k1, then k2 "0, h,q cos a > (k2,,,, h, + k22,e, h2) q sin a.

If in the foregoing k2, > k" k22 < k1, and k2, ml + k2.2 rel = 0, then the transverse relative thermal expansion co-efficient k2 - k1 in this direction is 0, and so is the transverse pressure force. If k2, > k, and k22 < k1, then a twoway thermal expansion mechanism is produced which gives a self-tightening thermal joint with an increase as well as a decrease in temperature.

In a fourth alternative possibility, assuming the part 3 to be rigid, greater values for a can be attained by adjusting the angle ss, the angle ss being that angle formed in the mem bers of the joint between the lengthwise direction d, e and the transverse direction d, c of the locking cavity. If the angle ss is sharp, then the part of the lengthwise thrust of member 3 will be deflected into a vector which runs counter to the transverse expansion force. If now the angle P is 45 , the forces are at equilibrium. If the angle ss is smaller than 45 , then the part 3 will be trapped in the corners formed by the angles ss, thereby producing a substantial decrease in the transverse pressure.This decrease in the pressure exerted in the transverse direction is paid for by an increase of the shredding tension against the oblique parts c, d. Since this requires increased mensuration, the described alternative cannot be applied to situations in which the size of the joint is limited.

It is to be noticed that the engagement of the joint depends solely on the parts 1, 2, whereas the locking of the joint presupposes only a pressure in the lengthwise direction d, e being generated in the case of, and to withstand, a deformation. This presupposition therefore can result either from an active pressure or from the presence of an impermeable body in the locking cavity. It is further to be noticed that the locking cavity c1, d,, d2, c2 forms a container which is closed in the plane abcdef. The consequence of this is that the locking may be achieved through an elastic member which is not however a fluid. A consequence of this is that the joint structure may be given a desired elasticity.

In Fig. 3, there is shown annular deformed member 3 in the locking cavity c1, d, d2, c2.

By adjusting the portion of contact and the wall thickness of the member 3, the joint structure can be given a desired degree of elasticity. Also the member 3 can be composed of a plurality of individual pieces, such as a cable as shown in Figs. 5 and 6. An important consequence of this latter arrangement is that the joint can form an extended curved seam.

If a curved seam is formed and it has a variable curvature, then the structure of the member 3 is not sufficient for the creation of the joint structure. Because of the locking cavity, the parts 1, 2 may be introduced to mutual engagement in two different ways.

They may be introduced in the direction s (perpendicular to the transverse sectional plane abcdef). Alternatively, they may be introduced perpendicular to the plane d1, d,', e1, e,' (d2, d2,, e2, e2'). In other words, the members 1, 2 may be placed along side each other as shown in Fig. 2 and then moved towards each other so that the surfaces b, c and d, e touch. They may then be moved in the lengthwise direction d, e until the locking oblique surfaces b, a and e, f touch.

With the first method of engagement, the curvature displacement t of the locking oblique surfaces a, b and e, f must be smaller than the lengthwise dimension c1, d2, (c2, d,) = q of the locking cavity. The above-mentioned condition clearly forcibly limits the curvature of the contact surfaces in the direction s and therefore also of the possible curvatures of the joint in the direction s.

With the second type of above-mentioned engagement of the parts 1, 2, the situation is different and the curvature in the direction s is clearly not limited by the conditions of the introductory movement. The parts 1, 2 can be brought together either if the angle which is formed between the diagonal c1, c2 and the lengthwise direction d, e is smaller than the angle a, or if the projection p' of the locking oblique surfaces e, f on d, e is smaller than q, the projecting straight line being parallel to ct, d, (c2, d2). Since the possibility of a seam with an arbitary curvature depends on the joining together of the parts of the joint, for a seam of an arbitary curvature, at least one of the conditions stated must hold.

With regard to the general properties of the joint, it is to be noted that the angles a, and a2 may also be unequal. Also, the joint may curve in the direction d, e, provided the curvature is constant as shown in Fig. 8.

Further, the direction d, e of the joint may substantially deflect from the general lengthwise direction of the parts of the joint as shown by Fig. 6. With one or more of these stated deviations, various advantageous shapes can be produced to give robust joints.

If desired, the shape abcdef can also form part of the overall shape of the joint, thereby embracing cut and rounded shapes. The lengthwise dimension q of the locking cavity can be used for creating a three dimensionally immobile structure.

If the contact surfaces have a faceted or curved shape (a, a", a', b, b", b', e, e", e', f, f", f') in the direction s as shown in Fig. 2, then the structure is also immobile in the direction s and is thus rigid in all directions.

The geometry abcdef can also repeatedly appear in the parts 1 and 2. In this case, the most advantageous arrangement is in the form of a graded ascent as shown in Fig. 7.

The member 3 can obviously be inserted by a force fitting, the locking into place being performed predominantly by the concave transverse surfaces c1, d,, (c2, d2) of the locking cavity c1, d1, d2, c2. The forcing into place may also be assisted by cooling the member 3.

The thermal joint of the invention enables a structural chain to be formed. The chain may be regarded as being equivalent to a continuous rigid body because the joints in the chain are immobile in three dimensions. As many of the thermal joints as desired may be employed in the structural chain. The thermal chain may exhibit a favourable reaction to vibration and to internal tensions of an extensive structure because such vibrations and such tensions can resolve themselves along the seams of the continuous structure. The structural chain may be mounted such that it closes in on itself. Then, if each joint of the chain is three dimensionally immobile and comprises contact surfaces with substantial values of the curvature displacement t, then the assemblage of the chain requires a simultaneous engagement of all of its constituent parts.

Referring now to Figs. 9 and 10, there are shown connections of a turbine blade 5 and a central disc 6 using the thermal joint of the invention. The diagonal c1, c2 (D) of the locking cavity cl, d1, d2, c2 is parallel to the radial direction r. Because of this, the blade 5 can be introduced radially and the contact surfaces (c c" c', f f" f') can be faceted as shown in Fig. 11 in order to produce a three dimensionally immobile structure. As a consequence, the blade 5 can be allowed a small movement which is controlled by the aperture of the angle a2 and the elasticity of the member 3.It is possible to give the connection of the blade 5 and the disc 6 a predomi- nately pivotal property, whereby the leeward side e1, f, accepts the tangential thrust of the outflowing turbine gases and the blade 5 can pivot and thus vibrate around the point f, in the tangential direction, the pivotal movement being controlled by the elasticity of the member 3 in the joint which is placed on the weather side only as shown in Fig. 9, and by the aperture of the angle a2.

For a one sided joint as shown in Fig. 9, the diagonal c1, c2 (D) may be parallel to the surface e1, f1, for the introduction of the blade 5 to take place. For a two sided structure as shown in Fig. 10, the direction of the diagonal D must be radial.

If the profile c c" c' (f f" f') in the blade 5 contains a slightly smaller angle Y than the corresponding angle in the disc 6, the resulting slight mensural error enables the structure to accept torque vibration. A more robust fitting may be achieved by having the joint on both sides as shown in Fig. 10, whereby more usual structures are approximated. The joint may of course be repeated giving a profile as shown in Fig. 7.

Referring now to Fig. 12, there is shown part of a rotor assembly in which the caps of the blades engage each other by means of thermal joints of the invention to produce a circumferential continuous chain. The chain is substantially equivalent to a continuous rigid body which in addition to serving the function of connecting the blades to the central disc can also serve the function of forming the rotor assembly.

Referring now to Fig. 13, there is shown a structural chain in which a thinner shroud can be produced than shown in Fig. 12. In Fig.

13, the geometry is placed in the cylindrical surface of the chain and the geometry forms a substantial angle f, with the tangential direction DT of the shroud, thereby introducing a more intricate shape a' abcdef f'. The shroud can be made even thinner by giving the portions a a' (f f') a simple overlap which in the front and back halves of the shroud goes in opposite radially deflecting directions. Increasing thinness of the shroud can be combined with a central circumferential ridge portion (not shown) for enabling better accommodation of the members 3.

Referring again to Fig. 12, there is also shown the pivotal connection of the blade and the central disc with the joint on the weather side and the joint on the lee side being fixed in position by overlaps 41 and 42 of the blades. As shown, the diagonals D (C1, c2) of the locking cavities are parallel to the radial directions r, thereby allowing substantial values of the curvature displacement t in a direction perpendicular to the cross-sectional plane abcdef (in the direction s) since the blades can be radially introduced.

In Figs. 9 to 1 2, the connection of the blade with the central disc is an axial type of connection usually employed in gas turbines and especially in jet engines. The constrained space available for the axially placed joint causes considerable restrictions. In Fig. 14, a structure is shown whereby these limitations can be avoided. More specifically, a central disc 6 is provided with a rim profile that is required of a part of the thermal joint. The blades 5 are provided with roots having a completive shape and they are placed side by side on the rim profile of the disc 6 to produce a thermal joint with the disc 6. The blades can also engage each other at their roots by the geometry abcdef being placed in a radial profile (as shown in Fig. 13) on the contact surfaces of the blades (not shown).

This is because the blades can be introduced radially, providing either the angle which is formed between the diagonal ct, c2 and the lengthwise direction d, e is smaller than the angle a or, if the projection p' of the oblique locking surfaces e, f on d, e, is smaller than q, the projecting straight line being parallel to C1, d1 (c2, d2), see the above-mentioned description with reference to Fig. 2. The space available in both the tangential direction as well as the radial direction is twice that of customary gas turbine arrangements.

Referring to Fig. 15, the space available for the roots of the blades in the above described arrangement with reference to Fig. 1 3 can be increased if the central disc is conical. The central disc together with the blade assembly can also adopt a spheroidal or pseudo-spheroidal shape.

In order to fix the last blade in position on the rim profile of the central disc, the disc may have, on two diametrically opposed places, a double profile with a graded ascent as shown in Fig. 7. A set of blades may then be provided with two blades which have longer roots than the rest of the blades, the longer roots extending over the double profile.

In this case, the two blades having the longer roots can be fixed after the rest of the blades and at the lower profile. When the chain of members 3 hidden in the circumferential channel formed by the successive locking cav ities of the blade assembly expands with a rise of temperature, the length of the chain increases. The members 3 then force themselves from both sides into the empty locking cavities on the upper joints (joints closer to the rim) of the two diametrically placed blades, thus fixing them at two points. The ejection of these thrust-in members 3 when dismantling the rotor assembly can be ascertained by two radial or axial grooves within root portions of the blades giving access to the locking channel.

The problem posed by the introduction of the last blade can also be solved by providing the rim with a double profile throughout and ensuring that the blades are divided into two sets, the root of every second blade extending over two profiles.

The third alternative is as illustrated in Fig.

1 5. In this third alternative, the central disc has a double conical structure and adjoining blades belong to different cones 6, and 62.

The cones 6, and 62 can be bound together using a thermal joint of the invention (not shown). With this arrangement, it is generally necessary to have the blades also engaging each other.

With the exception of the case where the central disc is a double cone, the situation is different according to whether the geometry in the rim profile opens to the leeward or to the weather side. In Fig. 14, the arrangement is shown as opening on the weather side. With regard to placing the geometry abcdef in the central disc in the most advantageous position in relation to stress, the geometry in the central disc may also be opened to the leeward side, especially if the transverse dimension of the central disc is limited.

When the rim of the central disc is profiled, arrangements can be introduced by increasing the reliability of the disc against thermal shredding, these arrangements being unavailable for known gas turbine structures. Fig. 1 6 shows the cross section of a rotor assembly having a central disc 6 with a rim profile required by the member of the thermal joint.

The rotor assembly also has a blade 5 having a complementary shape and creating with the rim profile the required type of joint. The disc is bound on the circumference by two thin annular rings 63 and 64 which have been force fitted on the central disc after having been heated whilst the central disc has been maintained at a low temperature or even cooled. Thus a structure can be formed which is efficient against thermal shocks.

There may be employed a circumferential band for a central disc with a type of joint which goes through the central disc in a substantially axial direction. This also gives another variant for a pivotal joint of the blade and the central disc. In this variant, the tails of the blades are responsible for the connection of the blades to the central disc, while part of the responsibility for creating an immobile structure is accepted by the circumferential chain around the central disc formed by the blade roots.

The thermal joint can also be used in structures which require the robust joining of parts together, especially of different materials, into an integrated whole which is capable of being machined in the joined state. The structure may form a pre-tensioned assembly which can be advantageous in circumstances such for example as when the structure receives thermal shocks. The structure may also exhibit improved wear characteristics because parts of it cannot become loose with wear. Indeed, with wear, the various parts tend to go deeper into each other, so creating a self-corrective assembly.

Once the angles a, and a2 are exactly alike, the geometry abcdef is able to engage itself and therefore both parts 1, 2 of the joint (and therefore the joint itself) can be produced with a single cutting device. The use of the single cutting device tends to ensure a perfect fit.

The concept of a structural chain which is equivalent with a continuous rigid body enables internal tensions in an extended structure to be relieved through a continuously discontinuous arrangement whilst retaining the properties of a continuous body.

If the blades in the central disc of a gas turbine rotor are joined together with the thermal joint of the invention, then the gas turbine rotor may exhibit properties which have hitherto only been possible in steam turbines. By employing the above-mentioned rim profile variant of the central disc, the thermal joint of the present invention can also be used in steam turbines. This gives the possibility of using steam turbines, or their structural equivalents, in applications in which it has only previously been possible to employ gas turbines. The thermal joint of the present invention also enables multiple binding structures to be employed in a turbine, thus giving better damage and tolerance factors and enabling loads to be divided between various independent systems.For example, if the continuous chain from the circumferential shroud is made conical, instead of its more usual cylindrical shape, then the pressure channels formed by the circumferential ridges on the outer surface create a gas bearing which is capable of accepting part of the axial pressure of the outflowing gases of the turbine. This pressure customarily is the sole burden of the joint between a blade and the central disc.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, for example, in so far as the invention has been described as being applicable to turbine rotors, it can also equally well be applied to turbine stators. Also, the relative faster expansion and the ensuing pressure may also result from a locking member 3, although being of a material with an equal thermal expansion coefficient to the parts 1 and 2, being forcibly cooled before inserting, and when attaining the temperature of the rest of the structure producing the necessary pressure through the whole series of temperatures which the structure may be subjected to.

The invention will now be defined by reference to the following claims. In the following claims, reference numbers and letters have been given to facilitate ease of understanding of the claims and it is to be understood that those reference numbers and letters are not to cause the claims to be construed in any limiting sense whatsoever.

Claims

1. A thermal joint comprising two parts (1, 2) which are of identical cross sectional shape (a1, bt, c,, d,, e1, f1, a2, d2, c2, d2, e2, f2), the two parts being engageable with each other to form a structure having a locking cavity (C1, d1, d2, c2), the parts (1, 2) having an equal thermal expansion co-efficient, and the locking cavity (c1, d1, d2, c2) containing a locking member (3) having a larger or a smaller thermal expansion co-efficient than the thermal expansion co-efficient of the parts (1, 2).

2. A thermal joint according to Claim 1 in which the shape of the locking cavity is rectangular, the surfaces of the locking cavity are planes, and the locking member (3) is a solid body, whereby the angles a formed between the oblique locking surfaces (e - f) and the lengthwise surfaces (d - e) of the joint are less then 45 .

3. A thermal joint according to Claim 1 in which the transverse surfaces (c1, c,", c,', d1, dill, d,', c2, c2", c2', d2, d2", d2,) of the locking cavity are concave in the direction (s) perpendicular to the transverse plane (abcdef), and in which the locking member 3 is a solid body, whereby for the angles a formed between the oblique locking surfaces (e - f) and the lengthwise surfaces (d - e), if Y is the angle in the direction (s) contained in the concave surfaces of the locking cavity, and s is the dimension of the thermal joint in a direction (s) perpendicular to the transverse plane (abcdef), then (hq + 2(s/2)2 cot(Y/2) cos(Y/2) sin(Y/2)) cos a > hq sin a.

4. A thermal joint according to Claim 1 in which the locking member is formed of a set of lengthwise slices (3" 32), the set falling into two sub-sets, the sub-sets having different thermal expansion co-efficients.

5. A thermal joint according to Claim 4 in which the thermal expansion co-efficients of the lengthwise slices (31 32) are such that some of the slices have a thermal expansion co-efficient higher than the thermal expansion co-efficient of the parts (1, 2), some of the slices have a thermal expansion co-efficient lower than the thermal co-efficient of the parts (1, 2), and the thermal expansion co-efficients satisfy the following condition which is that if the thermal expansion co-efficient of the parts (1, 2) is k1 and the thermal expansion coefficients of the two lots of lengthwise slices are respectively k2 1 and k22 and if k2 1 ml = k21 - k1 and k22,e, = k22 - k1, then k21,e, + k2.2rel = 0.

6. A thermal joint according to Claim 1 in which the locking member is of an annular or deformed annular shape.

7. A thermal joint according to Claim 1 in which the locking member is formed of a set of round or angular rods.

8. A thermal joint according to Claim 1 in which the lengthwise surfaces (c1 - d2, c2 - d1) of the locking cavity are convex in the direction (s) perpendicular to the transverse plane (abcdef).

9. A thermal joint according to Claim 1 and substanially as herein described with reference to the accompanying drawings.

10. A turbine structure comprising a thermal joint as claimed in any one of Claims 1 to 9.

11. A turbine structure according to Claim 10, the turbine structure being a turbine rotor having a central disc and a circumferential set of blades, the blades either engaging each other to form a self-closing circumferential chain or the blades being connected to the central disc by a thermal joint as claimed in any one of Claims 1 to 9.

1 2. A turbine structure according to Claim 10, the turbine structure being a turbine rotor having a central disc (6) and a set of blades (5), the central disc having a rim profile with a cross section (a1, b1, C1, d1, e1, f1) as desired of a member of the thermal joint, the root portions of the blades having complementary shapes, and the blades being positioned on the circumference of the central disc, thus creating by repetition a circumferential continuous thermal joint with the central disc.

1 3. A turbine structure according to Claim 10 and substantially as herein described with reference to the accompanying drawings.

14. A structural chain comprising a continuous rigid body having a chain of components engaging each other by a thermal joint as claimed in any one of Claims 1 to 9, the joint being continuously repeated.